The ultrastructural and cytochemical features of synaptonemal complexes (SC) in sections of spermatocytes of Mus musculus were studied under electron microscope. In specimens stained with uranyl acetate and lead citrate the SC was found consisting of three main elements. the lateral element (LE), the central element (CE) and the transverse filament (L-C filament). When stained with the Bernhard's technique, the SC was recognized as a contrasted, tripartite structure which was usually located in the bleached area occupied by the condensed chromatin and composed of highly electron-dense LEs and medium electron-dense CE and L-C filaments. The SC and the LE, stained either by uranyl acetate-lead citrate or by the Bernhard's technique, always showed diameters of about 210 nm and 60 nm, respectively. The results suggest that RNA may be an important component of the SC.